In the afore-mentioned copending application I have pointed out that monocrystalline wafers are of considerable significance in the production of semiconductive circuit elements in electronic technology. The production of such silicon wafers generally involves the lowering of a high-purity monocrystalline silicon seed into a melt of elemental silicon contained in a quartz crucible or vessel which must, because of its sensitivity to the temperature at which the melt must be held, generally be supported in a graphite or carbon outer vessel.
Various techniques are utilized in the generation of bars and the process may be carried out under a controlled atmosphere or vacuum to minimize the contamination of the melt. That application also points out that the quartz crucible tends to soften or contribute contaminants to the melts, that contaminants may enter the melt as a result of transfer from the supporting vessel, and that the stability of the silicon may be adversely affected by the inductive methods used to generate the heat forming the melt.
It is there also pointed out that deformation and deterioration of the crucible can be detected in part by coating the crucible as described in the commonly assigned copending application Ser. No. 614,434 filed 25 May 1984 and entitled METHOD OF COATING CERAMICS AND QUARTZ CRUCIBLES WITH MATERIAL ELECTRICALLY TRANSFORMED INTO A VAPOR PHASE.
That application is a continuation-in-part of my application Ser. No. 494,302 filed May 13, 1983 as a continuation-in-part of Ser. No. 358,186 filed March 15, 1982 (U.S. Pat. No. 4,438,183), in turn a continuation-in-part of Ser. No. 237,670 filed Feb. 24, 1981 (U.S. Pat. No. 4,351,855) incorporating subject matter of disclosure document Nos. 078,337, 078,334 and 078,329, all of Feb. 26, 1979, and disclosure document No. 082,283 of July 5, 1979.
The earlier application dealing with the quartz crucibles describes the formation of silicon carbide and silicon nitride in the vapor phase in situ upon the vaporization of elemental silicon and the coating of the quartz crucible with silicon and these compounds.
Application Ser. No. 618,192 recognizes that some of the problems remain in spite of the coating because of the softening action of the quartz crucible.
Consequently, Ser. No. 618,192 provides a unique approach to eliminating contamination of the melt and the silicon which is produced therefrom, utilizing the discovery that silicon granules or powder (generally: particles) have a relatively low conductivity, both thermal and electrically, and thus it is possible to effect an electric melting in a mass of such particles of the silicon by generating an electric arc within this mass and thereby allow the remainder of the mass to form an insulating layer between the crucible and the melt which is thereby produced.
For especially high purity production of silicon melts, however, this method may not be fully satisfactory since there is always a possibility of migration of contaminants through the interstices of the porous mass, the contamination of the surfaces of the particles, or the transfer of contaminants to the melt via the insulating layer.